In the automotive industry, BMC molding compounds can be used to fabricate various parts, in particular, highly stressable cylinder-head covers and oil pan covers, rear flaps, vacuum manifolds and/or bumpers. BMC molding compounds are, of course, useful in other applications such as the manufacture of household appliances, toys, furniture, etc.
The physical and mechanical properties of BMC molding compounds can be adapted to specific applications by the selection of the resins, additives, setting agents, fillers and reinforcement fibers, used in their formulation. A detailed description of conventional formulation materials and their effects may be found in the Plastics Manual [Kunststoff-Handbuch], 2nd edition (1988), pp 288 to 291. This reference describes the current state of the art and the descriptions therein concerning SMC technology (manufacturing flat work pieces) also apply to BMC technology.
Thermosetting and thermoplastic plastics shrink (contract in volume) when they are solidified in closed molds, e.g., extrusion, injection molding tools. As a result when cooled, the dimensions of the resulting molded parts are smaller than the corresponding dimensions of the tool. To reduce this contraction effect, Low Shrink (LS-) and Low Profile (LP-) unsaturated polyester resins are employed to reduce shrinkage and obtain molded component surfaces which are qualitatively good. The LP- resins are used as additives to control shrinkage in reinforced unsaturated polyester molding compositions that are cured at temperatures above about 100.degree. C. The LP- additives are typically dispersed or dissolved in a monomeric solvent, e.g., styrene, before being added to the unsaturated polyester compositions.
The contraction-compensating effect of LP-additives is believed to result from the fact that the thermoplastic separates out of the resinous system as a dispersed phase during the cross-linking process and consequently the monomeric styrene is occluded or dissolved in the unsaturated polyester resin. Initially the occluded styrene monomer does not take part in the copolymerization of the dispersed phase. The free styrene develops a vapor pressure corresponding to the curing temperature and the heat of reaction being liberated. This increases the volume of the cured dispersed phase thereby compensating for the contraction caused by cross-linking the unsaturated polyester resin. After the unsaturated polyester has formed a compacted (cured) outer phase the remaining styrene polymerizes. LP- additives are known to reduce shrinkage of conventional unsaturated polyester resins to between about minus 0.04% and plus 0.04% by volume. Notably, the contraction associated with unsaturated polyester resins can be overcompensated for with LP-additives and even produce expansion.
In comparison, LS-(Low Shrink) additives are known to reduce contraction to about minus 0.06%. Therefore, LS-additives are not well suited for melting core technologies where resins are shrunk onto a core so intensively that they often crack. For this reason, unsaturated polyester resins having LP- characteristics are preferred for use in melting core technology applications. A more detailed description of LS- and LP- resins can be found in the Plastics Manual supra the text of which is incorporated herein by reference.
Intense internal forces (stress and strain) in molded components can be caused by vibration, e.g., vibration produced by a chassis and/or motor, particularly in the case of diesel driven vehicles and equipment. These internal forces can develop in complicated molded components, such as vacuum manifolds and motors, casings and sheathings, leading to physical failures. The stress and strain inherent in such components can also lead to failure, e.g., crack formation and breaking, so that a small overload from external forces can be enough to produce destructive bending and/or shearing forces.
For the above reasons, unsaturated polyester resins having LP- characteristics have been used to manufacture Components for such applications. However, a significant disadvantage of these LP-modified unsaturated polyester resins is that they are not easily dyed uniformly due to a `white effect` known to occur during hardening, i.e., striae and spots form on the surface of the cured component. In addition, not all coloring pigments, particularly the black or white dyes, can be used for dyeing in LP- unsaturated polyester resins, because clear, uniform color tones cannot be produced due to the two- phase system which occurs in these resins (Plastics Manual (1988) pp. 290).
Dying of BMC molding compounds has become increasingly important, particularly in the automotive industry, where the appearance of motor parts is a design requirement. For example, a deep black coloration for vacuum manifolds is often desired. Moreover, the production of heterochromatic engine components, such as couplings or casings, may be important in the future of the automotive industry. Heretofore, for the above mentioned reasons, the prior art has not provided the desired uniform dyeing of thermosetting polyester BMC molding compounds having LP-characteristics.